Friday, 23 March 2018

Electronics - Long range low power wireless project.


Livestock Collar using long range wireless - A description of the work I was involved in for Hoofprints Technologies Ltd. (2016 to 2017)


Summary;

In built up areas LoRaWAN's range may be only 100's of metres although it can bounce into caverns and reach places you may not expect. In one of the farms in Britain the Hoofprints livestock collar is being trialled the range is over 3KM and the radio is operating nearly at the highest data rate, SF=7.

Earlier experiments the spread factor was fixed at SF=12, the slowest data rate, and with a poor aerial at a range of 2KM. In that earlier experiment radio path seemed to be over a small hill and bounced off other mountains, giving a patchy but better coverage than might be expected. These cases each with a single gateway. More gateways are recommended in order to give a more uniform coverage.


Top View - the NFC coil is mounted on the pins at the left hand side below

The Wireless Livestock Collar;
The unit has three radio's;
  • LoRaWAN 868MHz - Long range low power radio. Using Spread spectrum modulation wireless, wide area network for data with security.
  • Bluetooth - Short range communication and used to process lamb to ewe pairing.
  • GNSS - Animal location uses Global Navigation Satellite Systems.
  • Plus NFC - For reading the units status and identity this operates regardless of battery condition.
The features include a motion sensor. The NFC is wireless powered so it also operates when battery has failed so the user can determine if the unit identity and status by NFC contact, regardless of the battery condition. An off-the-shelf NFC ferrite coil made by Coilcraft has been used.

General Description;
It has been found that the unit works very well. LoRaWAN is operating at high data rate with a smallest spread factor of SF=7 giving 5.5K bps [bits per second] effectively over a distance of 3KM. The anticipated battery life is two years.

 LoRaWAN is a robust low power, low data rate, long range wireless
 protocol running. on 868MHz in the UK mostly there are other bands. 
The LoRa modulation is spread-spectrum modulation which the 
carrier centre frequency follows a zig-zagging waveform within one 
 channel within the band, I understand. The improvement comes about 
because the carrier moves through quiet and noisy parts of the channel 
consequently enough of the packet gets through usually. Even if two 
packets collide one should still get through. The detail of this 
implementation of spread spectrum is owned by SemTech, USA, who 
make the radio integrated circuits. 
Top view looking through the circuit board of 
the external layers through to the under side.
1. GNSS module - Telit SL871L or Ublox Neo modules were comparable the choice was made on manufacturer/distributor support for the modules and the aerial tuning and also support for the RM-186 Laird LoRaWAN module.

2. There are some Inter-Integrated-Circuit bus (I2C) peripherals. The Global Navigation Satellite System module was powered off between use is an I2C peripheral. A power supply remains connected to the back up pin of the GNSS module so that navigation data is retained.

The circuit below separates the powered peripherals side of the I2C bus from GNSS module when its power is turned off.
 I2C bus is isolated from the GNSS module 
when that module is powered down.

3. The patch GNSS aerial is placed away from the edges of the board giving it an area of 0V plane surrounding it. There are pads for clips to mount a screen can over the GNSS Patch aerial connection and matching network. Unfortunately there was a difficulty in fitting these clips but in any case the can was not necessary because the 868MHz cable could be routed away from that area.

The aerials are placed at least 1/8th wavelength apart on top of an 0V plane. The Bluetooth aerial is integrated into the RM-186 Laird wireless module. The White area top right has no power planes or circuits tracks as specified by the data-sheet for the module.

4. The LoRa RF is signal is routed by cable to the Bead 868MHz aerial made by RF Solutions. Please note that I have placed many vias coupling 0V planes on 3 of the four PCB copper layers.
  • The board is higher density 4 layer 90mm x 45mm whereas the previous board was double sided (100x52mm) giving a safer larger 0V plane area for the aerials that has proven to be not necessary. The design has three radio's; LoiRaWAN 868MHz, Bluetooth and GNSS. Plus NFC provided by a Coilcraft wound coil mounted on the pins on the bottom left side of the board and facing to the left of the unit.
  • Top and bottom have 0V planes with very many vias plus 0V internal plane and 3.3V internal plane has some tracks.

5. NFC - this is a Coilcraft NFC ferrite mounted on pins so that the side of the unit can be touched. It is passive and does not require battery power so the unit will report the last battery voltage before it shut down safely.



Battery protection + SII  S13R1 series 
LDO regulator has bias current of <9uA

6. The lower area of the PCB includes a low bias linear power supply with reverse current protect to prevent current from the debug lead flowing into the battery. It also has Lithium cell under voltage protection and reverse connection protection. The reverse protection provided by a P-channel MOSFET to form a super low drop pseudo diode.
GNSS power supply is separated from the 
main power supply. This is so that the main 
power supply remains relatively free of dips 
when the GNSS is switched on.
Conclusion;
With care UHF and microwave radio has turned out not to be as frightened as I had thought to work with. This board went through a number of reviews and advice was sought and taken resulting in the board worked first time. There some changes to carry out they are not urgent but for a connector part number mistake in the BoM. The requirement did change and so their were earlier boards purchased and designed.

I am told of and I have seen examples of PCB designs where the same care has not been taken, such designs perform poorly. All design work needs care be taken then you can expect very good outcomes.

1 comment:

  1. The link below is too a good explanation of LoRa spread spectrum modulation. There are other types of spread spectrum such as frequency hopping.

    https://youtu.be/T3dGLqZrjIQ

    ReplyDelete